test(vereinfacht): add tests and scope README

- Config, Client, SyncContact, Vereinfacht module tests (no real API)
- vereinfacht_test_README: document test scope

test/mv/vereinfacht/vereinfacht_test.exs

@@ -0,0 +1,59 @@
+defmodule Mv.VereinfachtTest do
+  @moduledoc """
+  Tests for Mv.Vereinfacht business logic.
+
+  No real API calls; tests "not configured" path and pure helpers (format_error).
+  """
+  use Mv.DataCase, async: false
+
+  alias Mv.Vereinfacht
+
+  setup do
+    clear_vereinfacht_env()
+    :ok
+  end
+
+  describe "sync_member/1" do
+    test "returns :ok when Vereinfacht is not configured (no-op)" do
+      member = Mv.Fixtures.member_fixture()
+
+      assert Vereinfacht.sync_member(member) == :ok
+    end
+  end
+
+  describe "sync_members_without_contact/0" do
+    test "returns {:error, :not_configured} when Vereinfacht is not configured" do
+      assert Vereinfacht.sync_members_without_contact() == {:error, :not_configured}
+    end
+  end
+
+  describe "format_error/1" do
+    test "formats HTTP error with detail" do
+      assert Vereinfacht.format_error({:http, 422, "The email field is required."}) ==
+               "Vereinfacht: The email field is required."
+    end
+
+    test "formats HTTP error without detail" do
+      assert Vereinfacht.format_error({:http, 500, nil}) ==
+               "Vereinfacht: API error (HTTP 500)."
+    end
+
+    test "formats request_failed" do
+      assert Vereinfacht.format_error({:request_failed, %{reason: :econnrefused}}) ==
+               "Vereinfacht: Request failed (e.g. connection error)."
+    end
+
+    test "formats invalid_response and other terms" do
+      assert Vereinfacht.format_error({:invalid_response, %{}}) ==
+               "Vereinfacht: Invalid API response."
+
+      assert Vereinfacht.format_error(:timeout) == "Vereinfacht: :timeout"
+    end
+  end
+
+  defp clear_vereinfacht_env do
+    System.delete_env("VEREINFACHT_API_URL")
+    System.delete_env("VEREINFACHT_API_KEY")
+    System.delete_env("VEREINFACHT_CLUB_ID")
+  end
+end